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        实现内置forEach循环
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    <script>
        function myForEach( arr , fn ) {
            for( var i = 0 ; i < arr.length ; i ++ ){
                fn( arr[i] , i , arr );
            }
        }
        var arr = ["张三","李四","王五","甘六"] ; 
        myForEach( arr , function( item , i , arr ){
            console.log( item , i , arr );
        }) ;
    </script>
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